There're no way to achieve what you want in native WordPress way. You have to use both
add_image_size() WordPress function and CSS to create resized thumbnail version of an image. @ialocin already explained what
add_image_size() does, so not repeating it.
First, you need to set a specific width, a very high value for height and set the hard crop mode to
false. Then set the dimension of the thumbnail using CSS. For example:
add_image_size('224_226', 224, 9999, false);
That will set the thumbnail width to 224px and stretch the height to maximum keeping the current proportion. Then add a class while calling the thumbnail..
the_post_thumbnail( '224_226', array( 'class' => 'image-224-226' ) );
Finally, set the height and width using the CSS..
You need to make sure that the height of uploaded image is bigger than the width, otherwise the thumbnail will have a shorter height but the CSS will stretch it to fit and you might get a distort image. The same applied for wider images too.